Rotary regenerative air preheaters are commonly used to transfer heat from combustion furnace flue gas to air used in the combustion furnace as combustion air. A conventional rotary regenerative air preheater 10, such as that illustrated in FIG. 1 includes a rotor 12 mounted within an interior 14a of a housing 14. The housing 14 defines a flue gas inlet duct 16 and a flue gas outlet duct 18 for a flow represented by arrow 20 of heated combustion furnace flue gas FG through the air preheater 10. The housing 14 further defines an air inlet duct 22 and an air outlet duct 24 for a flow represented by arrow 26 of combustion air CA through the air preheater 10. The rotor 12 includes a plurality of radial partitions 28 or diaphragms defining compartments 30 therebetween for element supporting baskets (frames) 32 of heat transfer elements 34. The rotary regenerative air preheater 10 is divided into an air sector 38 and a flue gas sector 36 by sector plates 40, which extend across to “cap” open top end 42 and open bottom end 44 of housing 14 to partially enclose rotor 12 within interior 14a of housing 14.
FIG. 2 illustrates an element supporting basket 32 including a few heat transfer elements 34 stacked therein. While only a few heat transfer elements 34 are illustrated in FIG. 2 for purposes of clarity, it will be appreciated that interior 32a of the element supporting basket 32 will typically be filled with multiple heat transfer elements 34. As such, heat transfer elements 34 are closely stacked in a spaced relationship within interior 32a of element supporting basket 32 to form passageways 46 between the heat transfer elements 34 for the flow of combustion air CA or flue gas FG therethrough.
Referring to FIGS. 1 and 2, the hot flue gas FG has a flow 20 through the gas sector 36 of the air preheater 10 transferring heat to the heat transfer elements 34 on the continuously rotating rotor 12. The heat transfer elements 34 in element supporting baskets 32 rotate about vertical axis 48, illustrated by arrow 50, out of gas sector 36 and into the air sector 38 of the air preheater 10. In air sector 38, combustion air CA has a flow 26 between the heat transfer elements 34. Combustion air CA is thereby heated by heat transfer elements 34. In other forms of rotary regenerative air preheaters, the heat transfer elements 34 remain stationary while the flue gas inlet duct 16/flue gas outlet duct 18 and air inlet duct 22/air outlet duct 24 of housing 14 rotate. For examples of other heat transfer elements 34, reference is made to U.S. Pat. Nos. 2,596,642; 2,940,736; 4,396,058; 4,744,410; 4,553,458; and 5,836,379.
Although known heat transfer elements exhibit favorable heat transfer rates, the results can vary rather widely depending upon the specific design. For example, while various undulations in the heat transfer elements may provide an enhanced degree of heat transfer, they also may increase pressure drop across the air preheater. Ideally, undulations in the heat transfer elements induce a relatively high degree of turbulent flow in the fluid medium adjacent to the heat transfer elements, while the fluid medium not adjacent to the elements (i.e., the fluid near the center of the passageways) experience a lesser degree of turbulence, and therefore less resistance to flow and less pressure drop. However, attaining the optimum level of turbulence from the undulations can be difficult to achieve since both the heat transfer and the pressure drop tend to be proportional to the degree of turbulence produced by the undulations. An undulation design that increases heat transfer tends to also increase pressure drop, and a shape that decreases pressure drop tends to also decrease heat transfer.
Design of the heat transfer elements must also present a surface configuration that is readily cleanable. In cleaning heat transfer elements, soot blowers are typically used to deliver a blast of high-pressure air or steam through the passages between the stacked heat transfer elements to dislodge any particulate deposits from the surface thereof and carry the particulates away leaving a relatively clean surface. To accommodate soot blowing, it is advantageous for the heat transfer elements to be shaped such that when stacked in a basket the passageways are sufficiently open to provide a line of sight between the elements. Such arrangement allows the soot blower jet of air or steam to penetrate between the heat transfer elements for cleaning thereof. Some heat transfer elements do not provide for such open channels, and although they may have relatively good heat transfer and pressure drop characteristics, cleaning using conventional soot blowers is ineffective. Heat transfer elements with open channels also allow for the operation of a sensor for measuring the quantity of infrared radiation leaving the heat transfer element. Infrared radiation sensors can be used to detect the presence of a “hot spot”, which is generally recognized as a precursor to a fire in the basket. Such sensors, commonly known as “hot spot” detectors, are useful in preventing the onset and growth of fires. Heat transfer elements formed and arranged without open channels prevent infrared radiation from leaving the heat transfer element thereby preventing detection by a hot spot detector.
Thus, there is a need for a rotary regenerative air preheater with heat transfer elements that provide decreased pressure drop for a given amount of heat transfer, provide surfaces cleanable by a soot blower and provide an arrangement compatible with hot spot detector use.